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Where does the power go?


Many years ago I got an free energy monitor from the power company and all was good. Then google closed their power meter project closely followed by me moving house. When planning to install some solar PV I decided it was time to dig out the power meter to get some baseline data before adding any complexity.

I reworked my perl script that scrapes data from the serial port of the energy monitor to push data into an RRDtool database file. From this data I generate a webpage containing some graphs and the most recent power figure.

What's the problem

I've now got a solar PV setup that feeds mains power generated into the consumer unit, any excess power is exported back out to the grid. I want to keep an eye on how much spare generation I've got that's being exported to the grid along with keeping an eye on my reduced grid power consumption.

Irritatingly the energy monitor I have pays no attention to the direction of power. The effect of this can be seen on the following chart of month data, the gap in data is during power off while the installation was being connected. Following the gap it looks like power usage goes up but it's actually the power flowing back to the grid!

For a close up of the problem here's data from a fairly normal day before solar power was installed

Followed by a similar day but with solar power in the mix!

What I'd like is to be able to log data with the correct sign so that I see power exported or imported on my graphs but right now I don't have that data.


  1. How do I measure power and direction
  2. Make a wife friendly display for this data (including logging it and making it available in web browser)

Next: Measuring power

Author: Graham Auld
Dated: 2nd August 2014